Imaging apparatus which employs the line-scan principle is well known. Typically, such apparatus will consist of a high energy radiation source such as an X-ray source, and for the purpose of exemplification herein further discussion will describe X-ray systems in particular. The beam of the source may be collimated into a curtain, usually referred to as a “curtain beam”, and is then detected by a linear array detector for example comprising a linear photodiode array. Image information is obtained by having the object of interest move linearly for example at right angles with respect to the beam and storing successive scans of X-ray transmission information derived from the linear array from which a complete image frame can be compiled.
If the object being scanned is heterogeneously transmissive of x-ray radiation, and for example consists of or contains multiple smaller objects and/or components of dissimilar materials, it can be possible to build up an image of the object, and in a particular case of the contents or components. The image may then be displayed on a viewing screen. This image can be useful for example in relation to the possible applications outlined above. In particular, it can be useful in determining the contents of a container or the internal structure of an object or body.
Even so, the image generated by such an X-ray apparatus is limited. At best it constitutes a two dimensional shadowgraph of the object being imaged. This can make it difficult to interpret.
European Patent No. 610084 describes a method of creating a “2.5D” solid model picture for viewing. A stereoscopic pair of X-ray images is obtained using two diverging curtain beams derived from an X-ray source. These are separated into conjugate slices and the 2.5D image built up from the resulting slice information.
The resultant image is not strictly a three dimensional image (although it is often so referred to) since it is presented on a two dimensional screen rather than by means of full stereoscopic apparatus. Such a 2.5D representation in fact contains psychological cues to depth such as linear perspective, interposition, shading and shadowing rather than the full physiological depth cue known as binocular parallax or stereoscopy which is required for a full three dimensional image.
The method of EP610084 still provides a user with a final image which can be rotated and looked at from different directions and which can give considerable information as to the relative positioning of different objects or compounds. However it does not give information as to the nature of the items which have been located.
UK Patent Nos. 2329817 and 2360685 are examples of methods and systems which can be used to produce full stereoscopic image pairs. They derive ultimately from principles set out in EP0261984. In particular they are subject to the condition set out at column 4 lines 31 to 48 therein which imposes considerable constraints on detector and source beam geometry. Although stereoscopic imaging can be a relatively powerful technique, exploiting full physiological cues in relation to depth information, and thus offering the potential for a user of the X-ray apparatus to identify objects or components much more readily and clearly, it can be complex in practical operation. To exploit the stereoscopic effect, it is necessary for the observer to receive different images at each eye simultaneously. This will necessitate the use of special apparatus. Moreover, a full stereoscopic technique requires precise control of the image collection process having regard to the conditions identified above. If the stereoscopic pair is to be effective, the respective images must be collected with a parallax that closely approximates to that which would be tolerated by the observer's eyes. For these reasons, full stereoscopic imaging has not gained wide acceptance for scanning machines of this type.
Not only do conventional non-stereoscopic apparatus and methods tend to give limited information in a third dimension, but also the images they produce also give limited information about the material content. In essence, at its simplest, all that is being measured is X-ray transmissivity.
In most practical systems even this is measured indirectly. At its simplest, a typical linear array detector comprises in combination a scintillator material responsive to transmitted X-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation. The detector merely collects amplitude information, and does not discriminate spectroscopically.
However, it is known that spectroscopic information from the transmitted X-rays could be used to give additional information about the material content of the objects or components being scanned. This has led to development of dual band detectors, which are capable of separately identifying low and high energy bands from the full spectrum of X-ray emissions. Such a dual energy sensor typically comprises a sandwich pair of semiconductor photodiode arrays or the like, in conjunction with a scintillator configuration that is configured such that the respective detectors detect transmission of low-energy and high-energy X-rays. It is known that the X-ray absorption properties of any material can vary spectroscopically, and that the amount by which the absorption properties vary depends in particular on atomic number. This is exploited by the dual energy detector to differentiate generally between objects having lower and higher atomic number elements predominating.
When exploited as part of a security or material identification system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. However, even such a system confers only limited information about composition. The organic/inorganic division is crude and approximate, can readily be confused by objects which are superimposed in the X-ray path, and will give no information concerning the crystalline or polycrystalline nature of an object.
UK Patent Nos. 2329817 and 2360685 incorporate dual-energy transmission detectors. Even so, the compositional information given by the arrangement is still limited, for example in that the low/higher energy duality effect can give only a crude approximation of an organic/inorganic split and cannot itself distinguish polycrystalline materials.
For this reason, the references include additional scatter detectors. X-rays are scattered by the materials they pass through and these scattered signals can contain information that may be used to identify the scattering materials. There is great applicability for these detectors as many of the materials that raise security issues, such as explosives, drugs and semiconductor materials, have a polycrystalline structure and therefore produce good scatter signals. This technique of identifying materials from the scattered signal though possible is not currently commercially used as the extra scatter detectors introduce greater complexity in the system and the scattered beams are weak and so throughput is limited.
The line-scan X-ray technique is widely used in security applications where the detection and differentiation of objects of complex and varied shape and composition is an important feature. A better resolution of the exact shape and location of such objects in three-dimensional space would be a considerable improvement on present techniques, especially if composition could also better be characterised.
It is an object of the present invention to mitigate some or all of the above disadvantages of prior art line scanning systems.
It is a particular object of the present invention to provide a method and apparatus for line scanning of objects, and especially of containers of multiple objects or objects comprising multiple components, which provides additional information about their shape and/or location in three-dimensional space.
It is a particular object of the invention to provide a method and apparatus which generates an image providing information about the shape and/or location of objects in three dimensional space which does not require special viewing apparatus but can be presented effectively on a two dimensional viewing screen.
It is a particular object of the invention to provide a method and apparatus which generates an image providing information about the composition of objects in three dimensional space to assist in the resolution of their shape and/or location in three dimensional space.
Therefore, according to one aspect of the invention there is provided an apparatus for generating and displaying an image of an object comprising:
a high energy radiation source such as an X-ray or gamma-ray source and a series of at least two but preferably three or more linear radiation detectors such as, as applicable, X-ray or gamma-ray detectors spaced therefrom to define a scanning zone therebetween, the detectors being capable of resolving incident radiation spectroscopically;
means to cause an object to move relative to and through the scanning zone in use;
an image generation apparatus to generate at least a first image from the output of a first linear detector, a second image from the output of second linear detector, and a third image, such that each such image includes a representation of spectroscopically resolved incident radiation; and
an image display adapted successively to display at least the first, second and third such images and thus display the monocular movement parallax between the images.
By analogy, in accordance with a further aspect of the invention there is provided a method of obtaining an image of an object comprising the steps of:
providing a high energy radiation source such as an X-ray or gamma-ray source and a series of at least two but preferably three or more linear radiation detectors such as, as applicable, X-ray or gamma-ray detectors spaced therefrom to define a scanning zone therebetween, the detectors being capable of resolving incident radiation spectroscopically;
causing an object to move relative to and through the scanning zone; and thereby
generating at least a first image from the output of a first linear detector, which image includes a representation of spectroscopically resolved incident radiation;
generating at least a second image from the output of a second linear detector, which has a different perspective from the first image; which image includes a representation of spectroscopically resolved incident radiation;
generating at least one third image which is also spectroscopically resolved;
displaying at least such first, second and third images successively and thus displaying the monocular movement parallax between the images.
Furthermore as an image can be created at different known energies it is possible to obtain images at many different energies. The monocular movement parallax effect can be created from each energy level or from a combination of many energy levels.
Thus, in accordance with the apparatus and method of the first and second aspects of the invention successive images are generated as relative movement is effected between an object and a source/detector arrangement. Suitable means are provided to effect relative movement of object and scanning zone, for example comprising an object handler. It may often be convenient for an object to be moved, for example by a suitable object handler or conveyor, through a static scanning zone, but it will be appreciated that it is also valid to have the object remain static and to translate the detector and source to create the relative movement.
At least three images are generated from at least one spaced pair (and preferably a series of three) linear X-ray or other radiation detectors. Two particular features of the invention operate co-operably to assist a user in resolving the images, and in particular in resolving multiple objects, components or parts of an image. First, although a plurality of images is generated, the images are not collected and displayed as paired images with the intention of providing a stereoscopic effect. Rather, the invention exploits the monocular movement parallax between the successive images. Second, each image includes a representation of the differentiated spectroscopic information collected by means of using linear X-ray or other radiation detectors capable of resolving incident X-rays or other radiation spectroscopically. The two effects reinforce each other in assisting in image interpretation.
Monocular movement parallax alone can give some effective physiological cues in relation to objects located in three-dimensional space, in particular if a successive multiple series of images is used, although the effect will be present for any pair of images. However, it has a number of advantages of simplicity over a full stereoscopic system.
In particular, the images are displayed successively as individual two-dimensional images, and require no special apparatus. The system exploits a monocular parallax. It is not necessary simultaneously to generate different images for each eye of a user as would be the case for stereoscopic viewing. A binocular or other complex stereoscopic viewing apparatus is not required. A simple monocular display is sufficient. Moreover, the relatively strict control of image production which is necessary to exploit binocular parallax effectively does not apply. For example for an effective binocular parallax pair of images, the images need to be generated in such a way that the parallax between them closely matches that which can be tolerated by a human observer. By contrast, an effective monocular parallax illusion of movement in three dimensions can be produced by a much greater range of successive images. It is not necessary to comply with the narrow conditions for beam geometry that apply to stereoscopic imaging in the prior art as set out numerically as noted above in EP0261984. The technique is thus potentially much more robust in practice.
The system and method in accordance with the first and second aspects of the invention are thus potentially simpler in many practical circumstances than a binocular method and apparatus, and, significantly, the invention tends to lend itself more closely to application with limited modification to existing apparatus and techniques. For example in the security field where X-ray line scanning using linear arrays of X-ray detectors is widely used, images produced in accordance with the method of the invention can be displayed on a two dimensional video display screen and information can be obtained from such images by a user seated at such a screen in an essentially conventional manner.
The monocular movement parallax effect is present in theory in just a pair of images, and visual cues in a third dimension which enable a user to resolve objects, and in particular individual contents and/or components thereof, being scanned can be obtained from such a simple pair of images. However, the effect is enhanced, and in particular a more effective impression of movement in three dimensions can be obtained, if a series of more than two successive images is generated and displayed. In accordance with the invention, at least three images are generated. More preferably at least five and more preferably yet at least six images are generated.
There are two ways in which additional images can be generated. First, there may be provided a laterally spaced series of more than two linear X-ray or other radiation detectors, for example at least three detectors, and more preferably at least five detectors. The output of each such detector may then be used singly to generate an image representative of data collected at the detector.
Where a plural series of laterally spaced linear detectors is provided, the space in between them is preferably generally constant, for example in that each pair of adjacent linear detectors in the series is substantially equally laterally spaced and/or at a substantially equal angular spacing relative to the X-ray or other radiation source.
The linear X-ray or other radiation detectors making up the laterally spaced series are preferably generally parallel. They may be disposed in a common plane or along a common arc. Each linear X-ray or other radiation detector may be of any suitable configuration, for example of a conventional simple or folded configuration.
Preferably the linear detectors making up the laterally spaced series are arranged such that the distance between them changes in order to maintain a constant angular separation between each array. This is of critical importance for L-shaped detectors which are commonly found in many scanning industries. This innovation is also of critical importance when the source of the radiation is not positioned at the central point of each array.
Preferably the apparatus comprises alignment means 4 to allow for precise alignment of each linear detector as above described.
Preferably the apparatus comprises means to deliver plural beams, preferably from a single X-ray or other radiation source. For example a beam is directed at each linear detector. Preferably the means to deliver plural beams allows a user to select a desired number of beams to correspond to the number of detectors in a plural array. Conveniently the apparatus comprises a collimator adapted to allow a user to select two to five beams from a single radiation source.
Additionally or alternatively, further images may be generated by interpolation of pairs of images collected from adjacent linear detectors. Thus, in accordance with the preferred embodiment of the invention, the method additionally comprises generating at least one intermediate image from at least one adjacent pair, and where applicable preferably generating at least one intermediate image from a plurality of adjacent pairs and for example each adjacent pair, of linear detectors, by processing the output of the or each said pair of detectors and generating an image representative of an output intermediate between the two said detected outputs.
The apparatus of the invention preferably includes an intermediate image generator comprising a means having this functionality.
Where a single intermediate image is generated between an adjacent pair of detected images, the intermediate image preferably extrapolates image points on the intermediate image to a mid-point between equivalent image points on the two detected images. Where a plurality of intermediate images is provided between a pair of detected images these are preferably based on a similar, evenly spaced extrapolation. Further preferably, the relative spacing of detectors and the configuration of intermediate image generation is such that all images are generated with approximately the same relative movement spacing. For example detectors are evenly spaced and the same number of intermediate images is generated between each pair of a plural array.
In accordance with the foregoing, either or both approaches is used to generate a series of images of an object in a scanning zone in what amounts, in effect, to different positions. In the first approach, a plurality of spaced detectors produce a corresponding plurality of direct images. In the second approach, actual data from adjacent linear detectors is used to generate an indirect image intermediate to the direct images, and thus representative of an intermediate object position.
Either or both methods may be used to generate a successive series of images from which, by means of the monocular movement parallax between each pair of successive images, information about a third dimension, and in particular an impression of movement through a third dimension, can be obtained. A relatively small number of images is required to produce an effective animation-like movement in three dimensions by a user observing a simple two-dimensional screen.
In accordance with the invention at least three images are generated. It will be appreciated in accordance with the foregoing that a third image requires either the use of a third detector to generate a third direct image or the use of the interpolation method to generate a third, indirect image from the output of a single pair of detectors. In practice both effects may be exploited together and a larger plurality of images might be preferred, for example at least three detectors generating at least five images, directly and by the interpolation method. However, a large number of linear detectors is not necessarily desirable and may require more complex apparatus. For example it might be desirable to provide five or fewer but to generate five or more and for example more than six images.
In a preferred embodiment therefore the intermediate image generator is configured such that five or more and preferably six or more images in total are generated from five or fewer detectors, and such that in any event more images are generated than there are detectors. In this preferred embodiment an apparatus comprises a laterally spaced series of no more than five linear X-ray or other radiation detectors, a direct image generation apparatus configured to generate an image from the output of at least one and preferably each linear detector and an intermediate image generation apparatus configured to generate at least one intermediate image from at least one and preferably each adjacent pair of linear detectors and adapted such that at least five and preferably at least six images are generated in total. In this preferred embodiment the method comprises generating a direct image from the output of at least one and preferably each linear detector; generating at least one intermediate image from at least one and preferably each adjacent pair of linear detectors, by processing the output of the or each said pair of detectors and generating an image representative of an output intermediate between the two said detector outputs; such that at least five and preferably at least six direct and intermediate images are generated in total
Sufficient intermediate images are generated to create at least five and preferably at least six images in total (that is, when direct images collected from the direct detected output of each detector and intermediate images generated by interpolation are summed). Where only two or three detectors are used additional images are created by synthesising more than one virtually spaced intermediate images between sufficient pair of detectors to result in the required number of images. The intermediate image generator is then configured to generate two or more intermediate images from at least one adjacent pair which show a gradual transition between the images produced by each adjacent pair by processing the output of the or each said pair of detectors and generating image representatives of a transition between the two said detected outputs.
Thus the embodiment generates at least at least five and preferably six images for optimum resolution of the parallax effect by a human viewer, but in all cases with no more than five linear detector arrays. Sufficient visual information is generated from a simplified apparatus.
The parallax effect is supplemented in accordance with the invention by a second effect that assists in resolution of different objects, components or parts of an image by presenting spectroscopic information differentially in the image (and thus presenting information having a relationship to composition differentially).
Prior art conventional scintillator-semiconductor detectors do not give any real spectroscopic information about the spectrum of transmitted X-rays. These simple detectors merely detect the presence or otherwise of transmitted X-rays. Even the dual energy detectors ultimately operate on the same principle, although they detect the presence or otherwise of X-rays within two distinct bands of the spectrum.
However, in accordance with the invention at least some of the linear detectors in the series comprise detectors that can generate spectroscopic information about the transmitted X-rays or other radiation. That is, the detector exhibits a spectroscopically variable response across at least a substantial part of the spectrum allowing spectroscopic information to be retrieved.
Proper resolution of spectroscopic information confers two advantages. It offers the potential directly to characterise the composition of different components or objects, or parts of the image, and by distinguishing between objects, components or parts of different composition, for example by representing them differently (such as in different colours) in the resultant image, it assists in resolution of different objects, components or parts of the image.
However, in accordance with the invention, spectroscopic resolution of transmitted X-rays or other radiation obtained from the output of each such linear detector is represented in the generated image. For example, spectroscopic differentiation in the collected data is represented in the image as differentiated colour, shading or marking. For example, a functional mapping is created between the spectroscopic information collected by the detector and the visual spectrum, and a coloured image display derived according to this functional relationship. Alternatively, a banded mapping is used in that the spectrum is divided into a plurality of bands, for example between four and eight bands, and different colours are used to represent each such band in the displayed image. The apparatus conveniently includes suitable image processing means to effect this mapping. Also as previously mentioned, images can be displayed either as single energy bands or as combinations of energy bands.
It is necessary that the detector system is enabled to detect radiation in a manner which is spectroscopically resolvable by the data processing apparatus. Preferably, a linear detector system in accordance with the invention, or some or all discrete detector elements making up a multi-element detector system in accordance with the invention, may be inherently adapted to produce spectroscopic resolution in that it exhibits a direct spectroscopic response. In particular a system or element is fabricated from a material selected to exhibit inherently as a direct material property a direct variable electrical and for example photoelectric response to different parts of the source spectrum. For example, the detector system or element comprises a wide direct bandgap semiconductor material. For example, the detector system or element comprises a semiconductor material or materials preferably formed as a bulk crystal, and for example as a bulk single crystal (where bulk crystal in this context indicates a thickness of at least 500 μm, and preferably of at least 1 mm). The materials making up the semiconductor are preferably selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide, thorium bromide. Group II-VI semiconductors, and especially those listed, are particularly preferred in this regard. The materials making up the semiconductor are preferably selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT) and alloys thereof, and for example comprise crystalline Cd1−(a+b)MnaZnbTe where a and/or b may be zero.
Combination of these and any other such materials may be considered which give spectroscopic detection rather than merely detecting amplitude of transmitted radiation.
The use of linear array detectors which confer full spectroscopic information in accordance with the invention is a significant enhancement as, in addition to the inherent advantages of giving compositional information from the spectroscopy, such an ability to identify materials will operate co-operatively with the ability to obtain third dimensional cues via monocular movement parallax to assist further in the resolution of multiple objects, components or image elements in the third dimension.
The basic geometry comprising at least two linear array detectors in serial array, and preferably a plurality of the same, to allow the generation of multiple images and to view these as an animated sequence is fundamental to the invention in providing an enhanced ability to interpret the shape and form of the object.
However, the greatly improved materials identification capability conferred by using spectroscopic linear detectors enhances both the ability inherently to identify composition of objects and the resolvability of different objects in an image via a number of steps which could be additive and reinforcing.
In particular for example spectroscopic detectors can be operated in an energy selective manner, giving rise to the ability to present an image resolved into a significantly increased number of X-ray or other radiation energy bands compared with the two that are available from standard prior art dual energy detectors. This information can be used to provide a much higher degree of material identification than is currently possible and also, by separately identifying objects of different compositions within a composite image, to improve resolvability of such objects.
As an additional effect, the way in which the geometry is arranged with multiple linear detectors arrayed in series across the scanning zone means that each of the ray paths seen by an individual detector is different. Again spectroscopic detectors will be able to gain much more information via these different paths. Some similarity can be seen to the multiple ray path technique used in standard CT technology.
By combining the effects of both the above two aspects of spectroscopic resolution using appropriate algorithms a much more precise interpretation of the type of material being scanned can be achieved.
Conveniently, the successive images are displayed under observer control, for example in that control means are provided to enable a user to display successive images under control, and for example at a chosen speed and/or in a forward and reverse order on the image display means, so as to create an effective manipulation of third dimensional cues from the image series. For example this allows a user to display successive images at variable speeds in order to enable a smooth transition for the human observer between each image and/or to run the images in a forward and reverse order. In accordance with the method of the invention the successive images are thus displayed at custom definable refresh rates and directions under observer control to facilitate interpretation.
The display means is conveniently a simple two dimensional display screen, for example a conventional video display screen (which term is intended to encompass any direct display or projection system exploiting any cathode ray tube, plasma display, liquid crystal display, liquid crystal on silicon display, light emitting diode display or like technology). It is a particular advantage that the method can be envisaged for use with, and the apparatus for the invention incorporated into, the standard display screens of comparable existing systems for example in the security of medical imaging fields.
The radiation source is in particular preferably a curtain beam source as will be familiar from conventional line scan apparatus. The source may comprise a single primary source adapted to generate a series of beams such as curtain beams aligned to be incident upon each linear detector in the laterally spaced serial array at a suitable angular separation, from example by a suitable beam splitting apparatus. Alternatively, multiple sources may be provided each generating a beam such as a curtain beam incident upon a linear detector in the serial array. The source may comprise a source combining both of the foregoing principles.
Each linear detector in the laterally spaced series is a detector for the detection of transmitted X-rays or other source radiation. For example, each detector is adapted to generate an electrical impulse in response to transmitted X-rays, thus enabling data to be collected from which an image can be constructed as an object is caused to move through the scanning zone. Each detector conveniently exhibits a photoelectric response and may for example comprise a linear array of photodiode cells.
In their primary mode of operation a laterally spaced series of linear array detectors is configured to receive multiple beams from the source which may for example be from a single X-ray or other radiation source. In a particular embodiment of this mode of operation, one or more sources may be used to generate a series of incident beams and for example curtain beams at relative angles appropriate to the distribution of the separate linear detectors. In this mode of operation, the method and apparatus of the invention may exploit the foregoing advantages.
The geometry also admits an alternative mode of operation. In accordance with this alternative mode of operation a single primary curtain beam is generated, for example directed generally towards a mid-point of the laterally spaced series of linear array detectors, and in particular in the direction of a linear detector at or about the mid-point of the series. The apparatus of the invention preferably includes a suitable collimator to collimate the output of the X-ray or other radiation source to produce such a primary beam.
With an object in place, this single primary beam is directed towards a central linear detector within the series of detectors. In these circumstances, this detector detects transmitted X-rays, but the other detectors could only be detecting diffracted secondary beams. Subject to appropriate calibration, signals that appeared on the secondary detectors would give additional information concerning scattered X-rays. It is known that scattering is characteristic of polycrystalline materials, which structure is relevant in relation to many of the explosive and like materials that a security detector apparatus might be directed at identifying. Accordingly, in this mode of operation, a significant enhancement of the detectability of such items is offered. This is achieved without the need for secondary scattering detectors but rather by suitable processing of data from the secondary mode of operation.
In a particular preferred embodiment, both modes of operation may be applied sequentially or closely successively or effectively simultaneously to collect maximum information. That is, images may be built up using a single primary beam directed through the full scanning zone created by the linear detectors in the series, and subsequently or closely successively or effectively simultaneously a single collimated primary beam may be directed at an individual linear detector, such as a generally centrally located linear detector in the series, and scatter information collected from the other detectors.
It is a particular advantage of the invention, since detectors are used which are capable of resolving information spectroscopically, that the data collected by these two methods can be resolved more readily to give substantial enhancement to the compositional information obtainable by operation of the system.
For operation of the method of the invention it is necessary to cause relative movement between an object to be scanned and the apparatus so as to cause the object to be scanned to move relative to and through the scanning zone. In practice for most practical purposes an object is moved relative to a stationary-scanning zone. For example the apparatus of the invention includes object conveyance means to convey an object to be scanned through a scanning zone. The object conveyance means may for example comprise of planar conveyor, which preferable conveys an object in a plane parallel to a plane in which the series of linear array detectors is disposed. For example, the conveyor is an endless belt conveyor or the like. However it is also valid to have the object remain static and to translate the detector and source to create the relative movement.
The source must produce a distribution of energies. Preferably the source is an X-ray source. Tungsten is the most appropriate target, but others could be used.
One or more of these features of the invention can be combined so as to provide enhanced information for an operator and the specific embodiment discussed below shows such combinations.